In cystic fibrosis (CF) airway disease, there is compelling evidence for two distinct clinical stages, an oxidative phase (early-stage CF) and an anaerobic phase (chronic, late-stage CF). Within the thick mucus lining the CF airways, the bacteria grow as a "biofilm," a form of development that affords organisms the luxury of enhanced resistance to antibiotics and biocides. The early oxidative phase is based upon a rapid and dramatic influx of neutrophils to the upper airways, an event triggered by bacterial infection. When stimulated, these professional phagocytes mount a potent "respiratory burst," an antimicrobial product from which is hydrogen peroxide (H202). In fact, neutrophils can generate millimolar levels of H202 within the phagolysomal vacuole. In contrast, H202 levels in blood are nearly 1000-fold lower. Surprisingly, a mutant of the major CF pathogen, Pseudomonas aeruginosa, lacking the H202-responsive transactivator, OxyR, is exquisitely sensitive to H202 and the bacteria perish even in the presence of blood H202 levels (micromolar range). Were OxyR to be compromised during human infection, bacteria would be unable to elicit a systemic infection because they would die via H202-mediated killing. Therefore, the goal of this proposal is to determine if OxyR of P. aeruginosa could serve as a drug target during various P. aeruginosa infections. The goals of this proposal are to (i) define a role for OxyR in animal virulence and resistance to human neutrophils, (ii) define the lesions that evoke exquisite sensitivity to H202 in the OxyR mutant and what OxyR-controlled gene products contribute to maximal or minimal protection, and (iii) determine whether OxyR is critical for survival of biofilm bacteria to H202 and aminoglycosides. [unreadable] [unreadable]